Initially, the signal processing of a television camera will be described with reference to FIG. 1. First, a subject image passes through a lens 2 of a television camera 1. The subject image is divided into three colors of R, G and B in a prism 3 and is converted to electric signals of R, G and B in imaging elements 4R, 4G and 4B. Thereafter, the electric signals pass through correlated double sampling (CDS) circuits 5R, 5G and 5B. The electric signals are amplified in variable gain amplifier (VGA) circuits 6R, 6G and 6B and are converted to digital signals in analog-to-digital (A/D) converters 7R, 7G and 7B. The digital signals are subjected to various kinds of processing in a video signal processing unit 8 and are then outputted as television signals from a video signal output unit 9. A timing generator (TG) 10 is a unit that generates timing signals for driving the imaging elements 4 and the CDS circuits 5. A central processing unit (CPU) 11, which is a system controller, controls circuits of the respective units.
A noise elimination circuit is included in the video signal processing unit. FIG. 2 illustrates a conventional noise elimination circuit. An input video signal is inputted to a low-pass filter (LPF) 12 and a subtractor 13. A high-frequency component (random noise) of the video signal is suppressed in the LPF 12. Then, the output of the LPF 12 is inputted to the subtractor 13. In the subtractor 13, the output of the LPF 12 is subtracted from the input video signal. The output of the LPF 12 is also inputted to a level determination circuit 14 where a coefficient A is calculated. In a multiplier 15, the output of the subtractor 13 and the coefficient A of the level determination circuit 14 are multiplied. The output of the multiplier 15 and the output of the LPF 12 are added in an adder 16, thereby generating an output signal.
The aforementioned operation may be represented by a formula: output signal=input video signal·A+LPF·(1−A), where 0≦A≦1.0. The output signal is obtained by mixing the input video signal and the LPF output at a ratio to be determined by the coefficient A. FIG. 3 shows the characteristics of the mixing coefficient A. When A=0, the output signal is equal to the LPF output, whereby a noise elimination effect is maximized. When A=1, the output signal is equal to the input video signal, whereby a noise elimination function is turned off. By appropriately setting a slope and a point, it is possible to control a brightness level, at which a noise elimination function works, and a noise elimination effect.
According to the conventional noise elimination function, as shown in FIG. 3, if the brightness level is lowered at a certain point, the noise elimination effect is gradually increased. In general, an image captured by a camera contains a portion where a brightness level is high. If one tries to obtain a noise elimination effect even in the portion where the brightness level is high, contour information is lost and the image becomes blurry. Thus, the conventional noise elimination function is applicable to only a portion where a brightness level is low and where contour information is intrinsically small.
Patent Document 1: Japanese Patent Application Publication No. 2010-200236